A typical conventional control apparatus for use with a water heater is provided with a microprocessor as its central control section. The microprocessor controls operations on various actuators including a gas solenoid valve for switching of the supply of fuel gas between ON and OFF states, a proportional valve for regulation of the supply amount of fuel gas, a fan motor for regulation of the supply amount of combustion air, a water flow rate control valve for regulation of the supply amount of hot water and other like actuating means.
PCT Publication No. WO 2006/080223 is intended for achieving improvement in safety of the operational control of such a water-heater control apparatus, and it discloses a control apparatus which has a main microprocessor and a sub microprocessor whose performance is lower than that of the main microprocessor. In the water-heater control apparatus, the detection signals from all of sensors are fed to both of these microprocessors. In this control apparatus, the main microprocessor executes control to supply hot water by the water heater, whereas the sub microprocessor executes control urgently to shut off fuel supply in stop conditions less severe as compared to the main microprocessor. In addition, generally, a microprocessor is equipped with analog signal input ports at which analog detection signals from various sensors are input, communication ports for establishing intercommunication with the other microprocessors and output ports from which control signals are output to various actuators.
In addition, for example, as disclosed in the JP-A-2007-322004, this type of water heater has a built-in freeze prevention heater for freeze prevention, and the control apparatus controls the heater for freeze prevention. Such freeze prevention control is achieved based on the detection signal from a specific temperature sensor adapted for measurement of the ambient temperature (e.g. the outside air temperature or the temperature of the inner space of the water heater), the combustion casing temperature or other like temperature.